As shown in FIG. 1, a conventional twin roll strip caster 100 feeds molten metal via an immersion nozzle 4 to form molten metal pool 5 in a space surrounded by two casting rolls 1 and 1a and edge dams 2 attached to both ends of the casting rolls 1 and 1a. Then, the strip caster 100 counter-rotates the casting rolls 1 and 1a so as to rapidly cool molten metal via heat flux into the casting rolls 1 and 1a owing to contact between the casting rolls 1 and 1a and molten metal, thereby producing a strip 6.
A meniscus shield 9 is disposed above the molten metal pool 5 for shielding molten metal from the open air. Gas inlets 8 are provided at both lateral portions of the meniscus shield 9 to feed inert gas to a surface of the molten metal pool 5. Brush rolls 7 are installed beyond the gas inlets 8 to brush the surface of the casting rolls 1 and 1a to remove foreign materials therefrom.
The strip 6 produced by the above strip caster 100 has a cross-sectional profile which is closely related to contours of the rolls in a casting space. It is most preferable that the strip 6 has a quadrangular cross section or a configuration with a slightly convex central portion so that it is finely rolled in a cold rolling or an after treatment to obtain a fine flatness of a final article. In order that the strip 6 may have such a fine configuration, edges of the rolls are straight or slightly concave at a-roll nip where the two casting rolls 1 and 1a are most adjacent to each other in the casting space.
In practice, however, the casting rolls 1 and 1a are heated to a high temperature during casting so that heat expansion causes the casting rolls 1 and 1a to be convex at their central outer peripheries although the central outer peripheries are straight when cooled down. Because the frozen strip has a cross sectional profile which accurately reproduces a cross sectional configuration of the casting space at the nip of the casting rolls 1 and 1a, the cross sectional profile of the produced strip is increased in thickness around the edges compared to the central portion.
Such a cross sectional profile acts a factor of a defective strip, which causes rolling defects in cold rolling, thereby degrading the quality and yield of a final article.
In order to compensate such heat expansion of casting rolls, as shown in FIG. 3, a casting roll 1, 1a is generally provided with roll crowns so that a middle portion b of the casting roll 1, 1a is flat or concave and both ends e thereof are concave. Although the crowns are formed in the casting roll 1, 1a, a strip 6 may be flat at a central portion B thereof but thicker at both edges E thereof, as shown in FIG. 4, owing to hot banding or bulging of molten metal from a central region of the strip 6 in a thickness direction. These edges of the strip 6 have a temperature higher than that of the central portion B. When a hot strip camera is used to photograph the hot strip under the roll nip between the casting roll 1, 1a, the edges are observed bright against the central portion as shown in FIG. 2.
If bulging or hot banding occurs at the both edges E of the strip 6 as described above, the quality and yield of the strip is disadvantageously degraded.
For the purpose of commercializing the Strip Casting (S/C) process, it is essential to develop a technology which can prevent the both edges E of the strip 6 from bulging or hot banding, thereby stabilizing the strip casting process while improving the quality and yield of the strip 6.
The above described methods for preventing the bulging of the both edges E in the strip 6 have been examined in various aspects by a number of inventors. In an early development stage of the S/C process, the inventors tried to prevent hot banding or bulging by adjusting the initial crowns of the casting roll and transversely differentiating the cooling ability of the casting roll since they believed that hot banding or bulging is caused by relative degradation in the freezing ability at the roll edges E.
For example, Japanese Laid-Open Patent Application Serial Nos. H6-297108 and H6-328205 disclose methods of adjusting the cooling ability by providing a plurality of cooling channels which are divided in a transverse direction. Japanese Laid-Open Patent Application Serial No. H9-103845 discloses a method of adjusting the quantity of roll crowns so that a central region in a thickness direction of a strip edge in a roll nip can have a solid fraction at a designated value or more. As yet another approach, Japanese Laid-Open Patent Application Serial No. H9-327753 discloses a method of adjusting the cooling ability in a transverse direction of rolls via differential procedures during surface treatment of the rolls.
The above conventional methods can more or less prevent bulging at both edges E of a strip in some casting conditions where casting roll 1, 1a of a strip caster 100 has identical specifications, steel are of equal type, or strips have the same thickness. However, there are drawbacks in that operating factors should be changed in response to variation of steel category, strip thickness, heat size and so on.
The assignee of the invention previously proposed to prevent hot banding owing to delayed solidification at strip edges as disclosed in Korean Laid-Open Patent Application Serial Nos. 1998-57611 which pertain to methods of adjusting the cooling ability of roll edges by feeding nitrogen gas, 1999-42986 which pertains to a method of regulating the thickness and composition of gas films on the surface of casting rolls, and 2000-79600 which pertains to a method of preventing inflow of abraded edge dam powder to lateral portion of casting rolls.
However, these conventional methods of adjusting the roll crowns, differentiating the cooling ability in a roll width direction and differentiating the surface treatment in a roll width direction have a fundamental problem in that they cannot actively cope with variation of steel types to be cast. These conventional methods also cannot overcome problems in that the aspect of hot banding is remarkably varied at both the strip edges according to the material of the edge dams or the type or composition of atmospheric gas and hot banding at both the strip edges becomes more severe even under equal casting conditions as casting time lapses, which is also called time dependency of hot banding.
In the meantime, FIG. 5 illustrates behavior of fluid existing around the casting roll. While this behavior is a typical phenomenon applicable to all kinds of fluid which can perform mass transfer under weak driving force, FIG. 5 illustrates factors which have direct influence on hot banding at both edges E of the strip 6 during actual strip casting. Those factors include an atmospheric gas such as nitrogen, externally introduced gas such as oxygen, ceramic powder abraded from the edge dams 2 due to friction between the edge dams 2 and end faces 14 of the casting roll 1, 1a, and fine oxide scale peeled off from the surface of the casting roll 1, 1a and the strip 6. FIG. 6 illustrates variation in build-up of abraded edge dam powder and oxide, which are deposited on edges and central portions of the casting roll surfaces upon completion of actual casting.
FIG. 5 schematically shows in its left part a simulation result of typical fluid behavior around the casting roll 1, 1a during rotation of the casting roll 1, 1a. Where the casting roll 1, 1a is rotated during casting, three different kinds of forces F1, F2 and F3 act on fluid around the roll surface, roll sides and a roll shaft 25 owing to centrifugal force. The driving force of these three forces are determined according to the rotation rate of a rotating body, physical properties of fluid and surface characteristics of the roll. Fluid concentration to the ends of the casting roll 1, 1a seems a general phenomenon in the rotating roll. Whereas, experimental results show that the quantity and the width W of fluid concentrating to the edges are determined owing to interaction among the driving forces F1, F2 and F3 having different directions from one another.
That is, the driving force F2 does not exist where fluid is not fed along the sides of the casting roll 1, 1a. Then, the driving force F3 gradually drives fluid on the roll surface toward the edges adjacent to the roll-sides so that fluid is built up around the edges. In case that fluid is continuously fed along the roll sides, the relatively large force F2 is generated so that fluid is concentrated to the edges. Then, the position or width of concentrated fluid is determined based upon the force balance between the driving forces F2 and F3.
The following will summarize influences of fluid to hot banding at both ends of the strip in strip casting:
First, the gas film thickness of nitrogen or atmospheric gas at the surface of the rotating body such as the casting roll 1, 1a, is not uniform in a width direction of the roll so that the both ends of the roll are relatively thicker than a central portion thereof to remarkably deteriorate the cooling ability of the roll. As a result, hot bands are created at the both ends of the roll where molten metal is not sufficiently frozen.
Second, the air directly contacts with the side of the rotating roll 1, 1a and the roll shaft 25, from which oxygen gas moves along a path b shown in FIG. 5 to the edge surface where it is built up. Because oxygen is expansible gas with a low solubility, it degrades close contact between a solidification shell and the roll as well as accelerates oxidation of the solidification shell. As a result, an oxide scale layer is additionally formed to degrade freezing ability.
Third, fluid having a large value of heat transfer resistance is continuously fed as fine ceramic powder is produced owing to friction between the edge dams 2 and the end faces 14 of the rotating casting rolls 1, 1a, a large quantity of roll surface oxide scale is formed by the brush rolls 7 which are mounted to remove roll surface pollutants, and oxide scale is detached from the strip. Such fluid is built up in the end portions of the casting roll 1, 1a to remarkably degrade the cooling ability between solidification shell and the roll.
As generally known, the boundary layer thickness 5 of fluid formed on a floating plate is proportional to the square root of a Reynolds number of gas as expressed in Equation 1,δ∝(υx/Vp)1/2  Equation 1,wherein υ is the kinetic viscosity of gas, x is the length of the plate from a leading end, and Vp is the moving rate of the plate.
The type of fluid existing between the casting roll 1, 1a and molten metal and the thickness of a film have greater influence on formation of the solidification shell. In casting of a thin film, heat transfer resistance controlling the heat flux between molten metal and the casting roll includes a casting roll body, a gas curtain between the roll and molten metal and oxide film or ceramic powder. The overall heat transfer coefficient between molten metal and the casting roll at a summit is expressed as in Equation 2,h=1/(dr/kr+dg/kg+ds/ks+dc/kc)  Equation2,wherein d is thickness, k is heat transfer ratio, subscript r is casting roll, subscript g is gas, subscript s is oxide film on the surface of molten metal, c is ceramic powder such as-oxide scale powder or abraded edge dam powder having a large value of heat transfer resistance.
It can be understood from Equations 1 and 2 that the overall heat transfer coefficient is varied by large values according to the type or composition of gas existing between the casting roll and molten metal, the thickness of gas layers, the type and thickness of oxide film and the type or thickness of abraded ceramic powder. The overall heat transfer coefficient rapidly decreases as the thickness δ of the gas film increases or the accumulation degree of an oxide layer or abraded ceramic powder increases.
That is, it is judged that bulging or hot banding owing to insufficient solidification occurs since fluid accumulating portions 16 at the both ends e of the roll have a heat transfer resistance between the roll and the solidification shell which is remarkably larger than that of the lateral middle portion b of the roll. The foregoing simulation result of typical fluid behavior tends to coincide with hot banding at both the strip edges in actual strip casting.
According to the foregoing three reasons, that is, thickness increase of the nitrogen gas layer at the both ends e of the roll, introduction of oxygen from the sides of the casting roll 1, 1a and local build-up of the heat transfer resistant particles such as oxide scale or abraded powder between the edge dams 2 and the end faces 14 of the casting roll 1, 1a, the cooling ability at the ends e of the roll are remarkably degraded compared with the middle portion b of the roll leading to bulging or hot banding owing to insufficient solidification. As the casting time lapses, the particles having high heat transfer resistant are increasingly built up at the ends e of the roll, thereby accelerating hot banding or bulging owing to delayed solidification.
The present invention has been made to solve the foregoing problems of the prior art and it is therefore an object of the present invention to provide an apparatus for controlling the gas layer thickness on casting rolls, which blocks introduction of heat transfer resistant particles in order to prevent bulging or hot banding owing to insufficient solidification or non-solidification at strip edges as well as compares the thickness of the gas layer at a central barrel portion of a casting roll with the thickness of the gas layers at the both ends of the casting roll, thereby effectively adjusting the cooling ability of the casting roll in a width direction of the strip.